1. Field of the Invention
One embodiment of the present invention relates to a carbazole compound. In addition, one embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each of which uses the carbazole compound.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively conducted. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Such a light-emitting element is of self-luminous type, and thus has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not needed, and so on. Therefore, such a light-emitting element is regarded as being suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission easily; thus, large-area elements using planar light emission can be formed. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element is very effective for use as a surface light source applicable to a lighting device and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, by application of voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby current flows. Then, these carriers (i.e., electrons and holes) are recombined, whereby the light-emitting organic compound is excited. The light-emitting organic compound returns to the ground state from the excited state, thereby emitting light.
Because of such a mechanism, the light-emitting element is called a current-excitation light-emitting element. Note that the excited state of an organic compound can be a singlet excited state and a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such light-emitting elements, there are a lot of problems that depend on substances, and in order to solve the problems, improvement of the element structures, development of the substances, and the like have been carried out (for example, see Non-Patent Document 1).